The present invention relates generally to chemical mechanical polishing of substrates.
An integrated circuit is typically formed by the sequential deposition of conducting, semiconducting or insulating layers on a silicon wafer. One fabrication step involves depositing a filler layer over a patterned stop layer, and planarizing the filler layer until the stop layer is exposed. For example, trenches or holes in an insulating layer may be filled with a conductive layer. After planarization, the portions of the conductive layer remaining between the raised pattern of the insulating layer form vias, plugs and lines that provide conductive paths between thin film circuits on the substrate.
Chemical mechanical polishing (CMP) is one accepted method of planarization. CMP typically requires that the substrate be mounted on a carrier or polishing head. The exposed surface of the substrate is placed against a rotating polishing pad. The polishing pad may be either a "standard" pad or a fixed-abrasive pad. A standard pad has a durable roughened surface, whereas a fixed-abrasive pad has abrasive particles held in a containment media. The carrier head provides a controllable load, in other words, pressure, on the substrate to push it against the polishing pad. A polishing slurry, including at least one chemically-reactive agent, and abrasive particles if a standard pad is used, is supplied to the surface of the polishing pad.
An effective CMP process provides not only a high polishing rate, but also a substrate surface which is finished (lacks small-scale roughness) and flat (lacks large-scale topography). The polishing rate, finish and flatness are determined by the pad and slurry combination, the relative speed between the substrate and pad, and the force pressing the substrate against the pad. The polishing rate sets the time needed to polish a layer, which in turn sets the maximum throughput of the CMP apparatus.
During CMP operations, the polishing pad needs to be replaced periodically. For a fixed-abrasive pad, the substrate wears away the containment media to expose the embedded abrasive particles. Thus, the fixed-abrasive pad is gradually consumed by the polishing process and, after a number of polishing runs (e.g., as few as about 40-50 and as many as several hundred), the fixed-abrasive pad needs to be replaced. For a standard pad, the substrate thermally and mechanically damages the polishing pad and causes the pad's surface to become smoother and less abrasive. Therefore, most standard pads must be periodically conditioned to restore a roughened texture to their surface. After a number of conditioning operations (e.g., as few as several hundred and as many as several thousand), the conditioning process consumes the pad or the pad is unable to be properly conditioned. The pad must then be replaced. An advantage of fixed-abrasive polishing pads is that they may not need to be conditioned.
One problem encountered in the CMP process is the difficulty in replacing the polishing pad. The polishing pad may be attached to the platen surface with an adhesive. Significant physical effort is often required to peel the polishing pad away from the platen surface. The adhesive then must be removed from the platen surface by scraping and washing with a solvent. A new polishing pad can then be adhesively attached to the clean surface of the platen. While this is happening, the platen is not available for the polishing of substrates, resulting in a decrease in polishing throughput. This problem is even more acute for fixed-abrasive pads, which need to be replaced more often than standard polishing pads. Thus, although the fixed-abrasive pads may not need to be conditioned, the use of fixed-abrasive pads in a CMP apparatus can result in a higher cost of operation.
Another problem that can arise when using a CMP process is difficulty in achieving a high throughput for the polishing process. Typically, substrates must be loaded and unloaded into the CMP apparatus. In addition to polishing the substrates using a fixed-abrasive pad, it is sometimes desirable to buff the substrates as well using a standard pas. The polishing, buffing, and loading/unloading steps are often performed sequentially using an architecture that performs only a single one of the steps at a time. It would, therefore, be desirable to modify the architecture of existing CMP apparatus to improve the overall throughput.